The amino acid and sugar compositions of four ribosome-inactivating proteins (gelonin, Momordica charantia inhibitor, dianthin 30 and dianthin 32) were determined. The proteins are all basic glycoproteins (pI greater than 8) containing mannose (more abundant in gelonin), glucose, xylose, fucose (absent from gelonin) and glucosamine. The ribosome-inactivating properties of the proteins examined are not modified by pretreatment with N-ethylmaleimide. Precipitating and inactivating antibodies can be raised against ribosome-inactivating proteins; a weak cross-reaction was observed only between dianthin 30 and dianthin 32.